Turbochargers typically include a turbine coupled to a first end of a mounted, rotatable shaft and a compressor coupled to a second end of the shaft. The compressor generally includes a compressor wheel enclosed by a compressor housing and the turbine generally includes a turbine wheel enclosed by a turbine housing. During use, exhaust gases produced by an internal combustion engine are used to drive the turbine wheel which, in turn, powers the compressor wheel. The compressor increases the pressure of the air entering the engine, so that a greater amount of oxygen can be provided for combustion than is generally possible with a naturally aspirated engine (this effect can be especially advantageous for diesel engines which often require an air-fuel ratio in excess of the stoichiometric limit for clean combustion). This increase in pressure provided by the compressor is commonly referred to as “boost”.
Turbines include turbine housings that are designed to contain the turbine wheel and to introduce a flow of exhaust gas to the turbine wheel. In some cases of high load (e.g., high engine speeds), if left unchecked, the energy of the flowing exhaust gas provided to the turbine wheel forces the compressor to produce more boost than the engine can safely withstand. This condition is termed “over-boost”.
Over-boost has traditionally been inhibited by providing an exhaust bypass or “wastegate.” The wastegate is designed to limit the energy provided to the turbine wheel by diverting a portion of the flowing exhaust gas away from the turbine (i.e., wastegating). Turbochargers provided with a wastegate, however, are usually sized for a low flow condition (e.g., a condition of low load or engine speed), requiring significant wastegating at or near rated power. Wastegating at rated power often results in poor fuel economy since the loss in mass flow throughput must be offset by an increase in expansion ratio across the turbine. As a result, the pressure upstream of the turbine rises, which leads to increased pumping losses for the engine.
Recently, variable turbine geometry (VTG) has been used to inhibit over-boost (VTG can also be used to create the adverse pressure gradient necessary to drive exhaust gas back to the intake manifold, a diesel emissions strategy referred to as “EGR”). VTG uses a set of movable vanes disposed in the turbine housing to control boost by altering the geometry of the turbine housing's internal space, thereby adjusting the pressure of the flowing exhaust gas introduced to the turbine wheel. While effective, VTG is often very complex and requires intricate control systems. The number of small moving parts, sensors, and controllers make them more expensive and more difficult to maintain than other turbochargers.
Improvements are continually sought for more efficient, cost effective, and adaptive turbochargers that provide sufficient boost control for both low and high load conditions.